I've read various scanned books that were from the 1910s and are out of copyright now. I read part of a book which may have been from the Motor Boys series, in which the boys, in their late teens, volunteered to fight in World War I. I'm not sure if it was called the Great War in the book, or if it started to be called the Great War later. In the book I'm thinking of, boot camp routines and drills were described in great detail. When the young men had a rest day, they put on a minstrel show, which was considered good clean fun at the time, at least by whites. There was prejudice against German-Americans at the time, too. It wasn't institutionalized in the same way racism was. I'll have to look up what book it was.
I read books from the earlier part of the twentieth century more critically now than I did when I was a child. I looked up reviews of Gene Stratton-Porter's books. I loved Freckles as a child, and a few years ago finally read Girl of the Limberlost. Some of the reviews of another book of hers, Her Father's Daughter, say that it's very racist, and then there are those reviews that say her views on the Japanese were correct. I'll probably read it for myself to see how bad the racism is, but apparently she uses the words "yellow peril" or has thoughts along those lines, so I'm not doubting it's racist. I've heard elsewhere that she was a white supremacist. The reviewers who are apologists say that that was perfectly acceptable in the 1920s. In a way that's hard to argue with, but there's actively being a white supremacist and passively going along with the system, and there were people who were anti-racist at the time. I wish I could say I was all that surprised that there are modern-day reviewers who think she was right, but I wasn't that surprised.
I loved Albert Payson Terhune's collie books as a child, too, but now they need warnings about the attitudes in them. I vaguely remember a story or two that had the "Jackson Whites" of the Ramapo Mountains in them, and not in a flattering way (http://www.goodreads.com/book/show/13421042-the-faith-of-a-collie). There was a lot of classism, too. I wonder if there are new editions that have some of that edited out or changed. I don't think those stories with the Ramapough Mountain Indians (Wikipedia says that's the modern name -- http://en.wikipedia.org/wiki/Ramapough_Mountain_Indians) could be edited enough. They'd now need a long explanation to go with them about racism, colonialism and indigenous people. And you'd need the explanation that at least a slim majority of Americans now, a century later, have different attitudes than that towards people who are mixed race.
(Added: I had forgotten the details in The Faith of a Collie. There's another group living in the mountains who are called "Blue-eyed N*****s." They don't get a line of dialogue, but several help the villain, and another is described as having his wife also being his aunt.)
It's funny that reading a little of a book that so far has sexism and classism led me to also consider racism in other books. I'm half-tempted to make an "intersectionality" tag, but not that tempted.